The bill’s mandate that individuals purchase health coverage “would imbue Congress with police powers rejected by the Founding Fathers,” Koster’s brief reads, while noting later he still favors “the expansion of health coverage.”

A subdued entry into an otherwise contentious fray, Koster’s brief–which delicately allowed the re-election poised Blue Dog to voice election-cycle dissent while still eschewing substantive legal action against his party’s health care reforms–puts him at odds with the Show Me State’s Democratic congressional delegation, most notably Senator Claire McCaskill.

Then making the case for the law’s adoption, McCaskill said in a 2009 interview that those who oppose the individual mandate are “frankly kind of dumb.”

On Tuesday, the National Republican Senatorial Committee highlighted the fissure in a release.

“In her own words, Senator McCaskill said she thinks those who oppose the individual mandate are ‘dumb,'” NRSC spox Chris Bond said. “[D]oes she think her fellow Democrat, Attorney General Koster, is dumb now that he has broken with her on this crucial issue?”

Though twenty-six states have joined Florida in challenging the constitutionality of Democrats’ health reforms, Koster only filed “friend of the court” brief. Koster, once a Republican state legislator, has been under increasing pressure to adopt a more aggressive posture towards the law, with many demanding he pursue legal action.

By nearly a three-to-one margin in a ballot referendum late last year, Missourians rejected the mandate found in the law. Separately, non-binding resolutions from the state legislature called on Koster to “[challenge] the constitutional and validity of the Patient Protection and Affordable Care Act.”

The brief is little more than an exercise of political expediency, reflecting the challenges awaiting red state Democrats next year, but the breach will no doubt rankle McCaskill’s reelection team.

First elected in 2006, McCaskill fashioned herself something of a dual identity, donning competing hats–a partisan fighter often deployed by the White House and a transparent maverick not afraid to buck party leadership–as appropriate.

But as moderate members of her own party defect on the merits of the president’s health reforms, McCaskill’s sustained defense of the measure might mean the end of her deeply guarded maverick reputation.